Free convection flows in vertical channels with two rectangular obstructions on opposite walls was studied numerically. Four different geometries were employed to study the effect of the parameters like Rayleigh number (Ra), aspect ratio (Ar), and obstruction locations (L(1), L(2)) on the average Nusselt number (Nu). The results included isotherms, streamlines and Nusselt numbers. Average Nusselt numbers for the obstructed channels were less than those of the smooth channel. The maximum reduction in heat transfer found in this study is approximately 31% and occurred at Ra = 5 x 10(2). The minimum reduction in heat transfer is approximately 3% and occurred at Ra = 10(4).